A number of Metropolitan Area Networks (MAN) and Wide Area Networks (WAN) utilize a Time Division Multiplexing (TDM) Synchronous Optical Network (SONET) network architecture, or a Synchronous Digital Hierarchy (SDH) network architecture. In contrast, a number of Local Area Networks (LANs) utilize an Ethernet network architecture.
The SDH/SONET standards evolved originally for use in a voice network. SDH is a European version of a standard that is substantially the same as the SONET standard developed in North America. SDH/SONET contains connection oriented synchronous TDM circuit switching technology. An SDH/SONET configured network runs at the same clock domain (e.g., every section of the network can be traced to a primary clock reference). The network allocates fixed bandwidth time slots for each circuit. The SDH/SONET architectures are connection based protocols in that there is a physical circuit arrangement between ports in a switch to establish an end to end path. The digital transmissions in signals traveling through an SDH/SONET network occur at the same rate, however there may be a phase difference between the transmissions of any two signals caused by time delays or jitter in the transmission system.
Ethernet evolved primarily as a data network. In contrast to SDH/SONET, Ethernet is a connectionless asynchronous Collision Sense, Multiple Access with Collision Detection (CSMA/CD) packet switching technology. The Ethernet architecture does not rely on a single clock domain like the SDH/SONET architecture. The Ethernet architecture sends a series of packets across the network containing data. Whenever a packet needs to be sent, the transmitter will try to transmit the packet. The Ethernet architecture is also connectionless in that the packets travel from node to node within the network without establishing a physical or logical circuit. The end to end path is discovered through a process called “Bridging”. Ethernet is fundamentally a Local Area Network (LAN) technology.
SDH/SONET networks provide reliable, guaranteed available bandwidth, low jitter connections. These characteristics are required for voice quality networks. SDH/SONET, however, is bandwidth inefficient and has a higher overhead than many other network architectures. Ethernet networks, in contrast, provide lower reliability best effort delivery, and low cost bandwidth connections. These characteristics are suitable for data quality networks. Ethernet has non-guaranteed transmission and low overhead, and supports fewer operational functions than SDH/SONET. In SDH/SONET, once the circuit is established, bandwidth is allocated for an application and cannot be used by any other application, even if the original application is not using the bandwidth. In Ethernet, applications only use bandwidth when they need the bandwidth to transmit packets.
A known approach of transforming Ethernet data traffic into SDH/SONET is to encapsulate or tunnel the data into SDH/SONET data traffic. Encapsulation entails taking data provided by a higher-layer entity as the payload for a lower-layer entity, and applying a header and a trailer as dictated by the protocol. The encapsulation process places the data payload into a frame for transmission.
A tunnel is a mechanism for encapsulating one protocol within another. A tunnel is used to transport information between devices using one form of communication across an infrastructure that only supports a different form, without translating from the first protocol to the second. Several standards are known for tunneling, such as RFC 2651, PPP over SONET/SDH, ITU-T Recommendation X.85/Y.1321, IP over SDH using LAPS, ITU-T Pre-published Recommendation X.86, Ethernet Over LAPS and IEEE Draft P802.3ae. There are also several proposals for Virtual Concatenation Extensions to SDH/SONET Standards, however no single standard has been established.
In encapsulation methods, a complete original frame is packed into an encapsulating frame. When this is done, overhead bytes of both frames are transmitted. This means that encapsulation is less efficient with regard to the use of overhead bandwidth. Current methods of encapsulation are also restricted to support only concatenated SDH/SONET frames. They provide only one single “pipe” to connect to LAN networks.
Current methods of encapsulation focus on converting Ethernet over SONET. Thus, the methods have the same bandwidth inefficiency problem that SDH/SONET networks have, in that bandwidth is allocated even if there is no traffic being transmitted over the SDH/SONET port.
Further, in encapsulation, both ends of the circuit have to be of the same port type. In addition, when encapsulating, the encapsulating protocol must support data field length larger than that of the protocol being encapsulated.